The Popular Brain Myth says that adulthood begins at age 25.

Let's think about this.

Our society prevents it from happening (as we should), but it is quite possible for a 12 year old to have a baby who grows up to have a baby at 12 years old and so you get a 24 year old grandmother. It's a bit less likely to get a 24 year old grandfather, but also biologically feasible. The claim that these people, who can literally be grandparents, are in fact children, is laughable :-D